1. Field of the Invention
The present invention relates to a display device in which a plurality of pixels are formed on an insulating surface and the luminance of the pixels is changed to display an image. Specifically, the present invention relates to a display device in which a driving circuit for controlling the luminance of pixels is provided on the same insulating surface where the pixels are formed.
2. Description of the Related Art
Display devices are incorporated in various electronic equipment. Reduction in size and power consumption is required for display devices, particularly for ones that are used in portable information equipment.
Display devices that are attracting attention for their reduced size and power consumption are flat panel displays such as liquid crystal display devices and OLED display devices using OLED (organic light emitting diode) elements.
These flat panel displays each have a plurality of pixels that form a matrix pattern on a substrate having an insulating surface. The flat panel displays form an image by using a driving circuit to selectively input a video signal to one pixel and change the luminance of the pixel.
There are various ways to connect pixels to a driving circuit.
As one way to connect pixels to a driving circuit for controlling the luminance of the pixels, the driving circuit is formed on a separate substrate such as a single crystal IC substrate and the substrate is bonded to the top face of a substrate having an insulating surface on which the pixels are formed. In this case, a large area is needed to bond the single crystal IC substrate to the substrate having an insulating surface on which the pixels are formed. In addition, the wiring resistance between the driving circuit and the pixels is large. Therefore it is difficult to provide a display device that is small in size and power consumption.
In another way to connect pixels to a driving circuit, the driving circuit is integrally formed on the same insulating surface where the pixels are formed. The driving circuit is composed of a thin film transistor (TFT) on the same insulating surface where the pixels are formed. This method can provide a display device that is small in size and power consumption.
FIG. 9 shows a top view of a display device having a pixel region in which a plurality of pixels form a matrix pattern and driving circuits that are formed in the periphery of the pixel region.
A source signal line driving circuit 902, gate signal line driving circuits 901 (901A and 901B), and a pixel region 903 are placed on a substrate 900 having an insulating surface. Signals to be inputted to the driving circuits (source signal line driving circuit 902 and gate signal line driving circuits 901) are supplied from an FPC substrate 904.
When viewed from above, a region of the display device excluding the pixel region 903 is referred to as frame. In other words, a frame in a display device corresponds to a region that does not display an image.
In a liquid crystal display device, the luminance of each pixel is determined by controlling the transmittance, which is achieved by controlling the orientation of each liquid crystal element. A liquid crystal element has a liquid crystal material placed between two electrodes. One of the electrodes of the liquid crystal element (hereinafter referred to as pixel electrode) is formed on a substrate on which a driving circuit and other components are formed (hereinafter referred to as pixel substrate) and the other electrode of the liquid crystal element (opposite electrode) is formed on a separate substrate (hereinafter referred to as opposite substrate). The pixel substrate and the opposite substrate are bonded to each other such that the pixel electrode faces the opposite electrode.
On the pixel substrate, a seal member is arranged so as to surround the pixel region and the driving circuits and then the opposite substrate is bonded. A liquid crystal material is sealed in the space surrounded by the pixel substrate, the opposite substrate, and the seal member. When the display device shown in FIG. 9 is a liquid crystal display device, 906 is the seal member used to bond the pixel substrate 900 to the opposite substrate. The opposite substrate and the liquid crystal material are not shown in FIG. 9.
In an OLED display device, the luminance of each pixel is determined by controlling light emission of each OLED element. An OLED element is formed on a pixel substrate after TFTs constituting a driving circuit and other components are formed. An OLED element is by nature degraded considerably by oxygen, moisture, and the like when it is brought into contact with the outside air. For that reason, the OLED display device employs a structure in which a cover member is placed after an OLED element is formed to shut the OLED element off of the outside air. The cover member is bonded to the top face of the pixel substrate using a seal member.
On the pixel substrate, the seal member is arranged so as to surround the pixel region and the driving circuits and then the cover member is bonded. The OLED element is sealed in the space surrounded by the pixel substrate, the cover member, and the seal member. When the display device shown in FIG. 9 is an OLED display device, 906 is the seal member. The cover member is not shown in FIG. 9.
It is common to various display devices including liquid crystal display devices and OLED display devices that the pixel region 903 has x (x is a natural number) source signal lines S1 to Sx arranged in parallel with one another and y (y is a natural number) gate signal lines G1 to Gy which are arranged in parallel with one another and which are perpendicular to the source signal lines S1 to Sx. Through the source signal lines S1 to Sx and gate signal lines G1 to Gy, pixels are selected and the luminance of the pixels selected is controlled.
The source signal line driving circuit 902 inputs signals to the plural source signal lines S1 to Sx. The gate signal line driving circuits 901 (901A and 901B) input signals to the plural gate signal lines G1 to Gy. The driving circuits 902 and 901 are formed in the periphery of the pixel region 903.
The source signal line driving circuit 902, which is composed of a shift register and other components, outputs signals sequentially in a scanning direction indicated by the arrow in the drawing. The signals outputted are inputted to the plural source signal lines S1 to Sx. Usually, the source signal line driving circuit 902 is arranged such that the scanning direction is perpendicular to the source signal lines S1 to Sx that are arranged in parallel to one another. Similarly, the gate signal line driving circuits 901, which are composed of a shift register and other components, output signals sequentially in a scanning direction indicated by the arrow in the drawing. The signals outputted are inputted to the plural gate signal lines G1 to Gy. Usually, the gate signal line driving circuits 901 are arranged such that the scanning direction is perpendicular to the gate signal lines G1 to Gy that are arranged in parallel to one another.
In FIG. 9, the gate signal line driving circuits 901 (901A and 901B) are formed to the left and right of the pixel region. Alternatively, only one side of the pixel region may have a gate signal line driving circuit.
The scanning direction of the gate signal line driving circuits 901 arranged as described above is called a row direction whereas the scanning direction of the source signal line driving circuit 902 is called a column direction.
In FIG. 9, the source signal line driving circuit 902 is formed in parallel to one of four sides of the rectangle of the pixel region 903. The gate signal line driving circuits 901A and 901B are each formed in parallel to one of two sides of the rectangle, which are different from the side parallel to the source signal line driving circuit 902 and which are not opposed to the source signal line driving circuit 902.
In this specification, of four sides of the pixel region 903 on the pixel substrate 900, the side connected to the FPC substrate 904 is called an upper side and the side opposing to the upper side is called a lower side.
Of four sides of the pixel region 903 on the pixel substrate 900, one side that is abutted with the side connected to the FPC substrate 904, and the side opposite to the one side are called a left side and right side of the pixel region, respectively.
Usually, the place of the source signal line driving circuit 902 is the closest to the area where the FPC substrate is bonded. Therefore the source signal line driving circuit 902 is generally placed above the pixel region 903. On the other hand, the gate signal line driving circuits 901 are placed to the left and right of the pixel region 903 on the pixel substrate 900.
Out of the four sides of the pixel region 903 on the pixel substrate 900, the source signal line driving circuit 902 may be placed on the side opposite to the side where the FPC substrate 904 is connected. In this case, the place of the source signal line driving circuit is below the pixel region.
It is assumed that above, below, left, and right of the pixel region 903 correspond to above, below, left, and right of the display device, respectively.
Users of portable information equipment such as cellular phones demand as large a screen as possible for displaying an image and reduction in width of equipment body so that it is easy to hold.
In order to obtain as large a display screen as possible and reduce the width of equipment body as much as possible, the area of the frame of a display device incorporated in the equipment body has to be reduced.
In the display device structured as shown in FIG. 9, the gate signal line driving. circuits 901A and 901B are placed to the left and right of the pixel region 903.
Furthermore, the seal member 906 is formed on the outside of the gate signal line driving circuits 901A and 901B on the pixel substrate 900. Accordingly, the area of the frame on the left and right of the display device cannot be reduced.
Also, the area of the frame can be reduced only to a limited degree on the upper and lower sides of the pixel region since the FPC substrate is connected.